Emiliania huxleyi is an ecologically important marine phytoplankton and it is regularly forms large blooms. Viruses are an important agent of mortality in blooms, however, little is known about the host:virus interactions in this system. This project investigated how infection at different stages of the host cell cycle influenced: virus adsorption, infection and production and host lysis and cell cycle. Cultures were infected during the G1 , S or G2 + M phase and monitored for 2 hours for adsorption and infection or monitored for 48 hours for virus production, host lysis and host cell cycle. A three fold increase in infection during the G2 + M phase was recorded, however there was no significant difference in virus adsorption or virus production between phases. Cultures infected in the G2 + M phase lysed faster than cultures infected G1 or S phase and there was a three fold increase in infection success if infected during the G2 + M phase. This may be due to changes in the host at the point of cell division. All cultures showed a deviation from the normal cell cycle activity after 24 hours of infection and remained in a stable, compromised state after this point. Virus production was independent of infection phase, this result may have been an artefact of the virulent nature of the system as the infected hosts began to lyse 4 hours after infection enabling a constant supply of viruses and repeated infections outside the initial phase of infection. The ecological significance of these findings is believed to maintain a variation in virus production and host loss which allows both components of the systems to perpetuate.

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